The invention relates to a luminescent alkaline earth metal orthosilicate activated by a rare earth metal. The invention also relates to a luminescent screen provided with such a silicate and to a low-pressure mercury vapour discharge lamp provided with such a screen.
It is known from British patent specification No. 544,160 that bivalent europium can be used as an activator in orthosilicates of calcium, barium and strontium. Dependent on the selected alkaline earth metal or alkaline earth metal mixture, emission in the green, yellow or blue part of the spectrum occurs upon excitation by ultraviolet radiation.
U.S. Pat. No. 3,260,675 describes the activation of orthosilicates of calcium and magnesium by terbium. Line emission in the green part of the spectrum takes place upon excitation by ultraviolet radiation. The Patent states that the terbium may be partly replaced by other rare earth metal activators, for example, samarium or dysprosium.
Activation of Ca.sub.2 SiO.sub.4 by dysprosium is known from U.S. Pat. No. 4,495,085. The luminescent material is intended for use in cathode ray tubes and emits white light upon excitation by an electron beam.
The activation of Ca.sub.2 SiO.sub.4 by cerium is known from the book by F. A. Kroger, "Some Aspects of the luminescence of Solids", 1948, page 288. The luminescence colour is blue.